Diethynylbenzene-ethynylpyrene copolymers

ABSTRACT

A high char yielding matrix resin for use in fabricating carbon-carbon composites comprised of a polymerizable mixture of a diethynylbenzene monomer and an ethynylpyrene monomer.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

This is a division of application Ser. No. 207,829, filed Nov. 17, 1980now U.S. Pat. No. 4,324,830.

BACKGROUND OF THE INVENTION

This invention relates to aromatic acetylene compounds and theirutilization as ablative materials for re-entry vehicles. In a moreparticular aspect, this invention relates to the synthesis ofdiethynylbenzene-ethynylpyrene copolymers and their use as high charyielding matrix resins for carbon-carbon composites.

The increased use and interest in the operation of re-entry vehicles hasgenerated a considerable research effort in an attempt to developstructural materials that exhibit high strength and resistance to thestresses and strains encountered by space vehicles during their re-entryregime. Rocket and missile components, such as turbine blades, nozzles,vanes, partitions, and especially nose cones, are very vulnerable to thestress and strain encountered during their re-entry environment. Thesecomponents require structural materials capable of surviving thosestresses and the elevated temperatures occurring during re-entry.

Graphite carbon, in the form of a carbon-carbon composite, using pitchas a resinous matrix, has been found to be useful in fabricatingstructural components. These materials possess many of thecharacteristics required by structural elements subjected to the stressof a high temperature re-entry environment. The carbon-carbon materialshave proven to be especially effective for nose tip applications andshow good thermal stress performance. Unfortunately, these materialsoften times do not show sufficient mechanical strength, discloseunpredicted anomalies in their ablation characteristics, and requireexpensive, high pressure processing techniques.

With the present invention, however, it has been found thatcarbon-carbon composites having superior re-entry characteristics can beproduced simply and easily by utilizing a novel aromatic acetylenecopolymer as a high char yielding matrix resin for the carbon-carboncomposite in lieu of the previously used pitch. The novel copolymermatrix resin is derived by effecting the copolymerization of a mixtureof diethynylbenzene and ethynylpyrene.

The reaction mechanism does not require high pressure processingparameters and the resulting copolymer chars easily when utilized as amatrix impregnant for graphite fibers. It easily wets the graphitefibers and penetrates into the pores of the fibers.

The present invention replaces the ill defined, variable compositionpitches utilized heretofore as an impregnant and matrix resin for thecarbon-carbon composites produced heretofore. The pitches are invariablycontaminated with S, O, N, P, ash, and other materials. In addition, thepitch is not homopolymerizable, therefore, it can exude from theimpregnated woven carbon composites during processing and, by virtue ofthe fact that it is not comprised strictly of aromatic hydrocarbon, hasa lower carbon content than the material of the instant invention. As aconsequence, pitch has a much lower char yield. The instant inventionprovides processing simplification far beyond the current state of theart pitch and provides more dependable performance characteristics forthe carbon-carbon composite products derived therefrom.

SUMMARY OF THE INVENTION

The present invention concerns itself with the synthesis of a novelaromatic acetylene copolymer derived from a mixture of diethynylbenzeneand ethynylpyrene and its use as a matrix resin for carbon-carboncomposites. The copolymers are unique in that they char very efficientlyin yields as high as 95%. Additionally, the chars are capable ofgraphitizing when heated to temperatures of 2400°-2800° C. Theprepolymer mixtures are very fluid when melted and, consequently, theycan readily impregnate a woven carbon fiber fabric. In addition, theyhomopolymerize when heated above 100° C. and, with a sufficientproportion of ethynylpyrene, the homopolymerization rate can becontrolled, and runaway reactions can be prevented. The novelty of thisinvention resides in the fact that it provides a material system whichyields high char, graphitizable, low viscosity, easy to process matrixresins for carbon-carbon composites. The composites are especiallyeffective for use as re-entry missile nose cones. The copolymer of thepresent invention has all the properties necessary to easily producehigh density carbons with minimal pressure requirements for fabrication.

Accordingly, the primary object is to provide an easily processablematrix resin for carbon-carbon components.

Another object of this invention is to provide a carbon-carbon matrixresin precursor of specific and known composition which can replacecurrently used pitch which has an undefined and never reproduciblecomposition.

Still another object of this invention is to provide a carbon-carbonmatrix resin precursor which can be processed at pressures below 500psi, thus eliminating the need for high pressures in the range of 15,000psi.

A further object of this invention is to provide a carbon-carbon matrixresin precursor which can cure without runaway exotherms.

Still a further object of this invention is to provide a carbon-carbonmatrix resin precursor which graphitizes efficiently into high densitygraphite.

A still further object of this invention is to provide a low melting(120° C.) carbon-carbon matrix resin precursor.

A still further object of this invention is to provide a carbon-carbonmatrix resin precursor which has a sufficiently low vapor pressure suchas to allow it to be processed without excessive loss due toevaporation.

A still further object of this invention is to provide a low viscosity,carbon-carbon matrix resin precursor which can effectively wet carbonand graphite fibers and fabrics and which can efficiently penetrate intothe pores of the fibers.

The above and still further objects and advantages of the presentinvention will become more readily apparent upon consideration of thefollowing detailed description of its preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with this invention, it has been found that theabove-defined objects can be accomplished by synthesizing novelpolyaromatic acetylene copolymers derived by effecting a reactionbetween diethynylbenzene and ethynylpyrene to produce a novel copolymerwhich can be effectively utilized as a high char yielding carbon-carbonmatrix resin precursor. The matrix resin precursor of this inventionreplaces the variable composition pitches utilized heretofore as acarbon-carbon matrix resin. In the prior art, pitch is procured by tradename. However, its composition varies from batch to batch since it is amixture of an innumerable number of naturally occurring compounds. It isnever pure, and contains undesirable elements of N, S, O, P and ash. Inthe present state of the art, pitch is used to impregnate wovencarbon/carbon fabrics at pressures up to 15,000 psi. The impregnatedmasses are then carbonized under pressure followed by graphitization upto 2700° C. Subsequently, they are reprocessed about 10 times untilmaximum densification is achieved. The necessary repetitious processingresults in a very high cost product even though the pitch component isvery inexpensive. Furthermore, contaminants such as S, O, N, P and ashcause the composite's erosion rates to be inconsistent.

The disadvantages of using pitch as a matrix resin, however, areovercome by the present invention. Specifically, the invention is amixture of two compounds whose structures are shown below. ##STR1## Byitself diethynylbenzene is too volatile, and polymerizes too rapidly topermit its use in the fabrication of carbon-carbon composites. Inaddition, its char is very difficult to graphitize even when heated to2700° C. However, it has two excellent advantages, in that the metaisomer is a fluid, and it produces a 94-95% yield of char whenpyrolyzed.

In contrast, ethynylpyrene is an excellent graphite former, but producesa lower char yield (45%) at atmospheric pressure. Its char yield can,however, be increased significantly if it is charred under pressure (upto 75-80% at 300 psi). Another advantage is its much slower thermalpolymerization rate and another disadvantage is the fact that it is asolid melting in the range of 120° C.

With the present invention, however, it has been found that a copolymersynthesized from a polymerizable mixture of diethynylbenzene andethynylpyrene has all of the advantages and none of the disadvantages.The mixture polymerizes at a controllable rate at temperatures in therange of 120° C., they char very efficiently (over 85%) afterpolymerization, and the chars graphitize efficiently in the 2300°-2800°C. temperature range. In addition, the mixture has a low meltingtemperature e.g., 60°-80° C. It is also fluid enough to allow it topenetrate deeply into the pores and intersticies of woven carbon-carbonfabrics. The mixture, thus, has a major advantage over the use of pitchor even the individual benzene and pyrene monomers as matrix resincomponents.

Generally, it is preferred to use diethynylbenzene: ethynylpyrene weightratios between 1:4 and 1:1, respectively. However, for some applicationsother ratios may prove to be preferable. Thus, ratios of 1:9 and 9:1constitute the range of ratios which have been found useful for thepurposes of this invention.

In utilizing the materials of this invention, the mixture of monomers isprepared and melted together to provide maximum homogeneity. It is thenimpregnated into a degassed woven carbon/carbon fabric. Whilemaintaining the pressure between 100 and 500 psi and heating at about100°-130° C. for 4-72 hours, polymerization is promoted. Whilemaintaining the pressure, the composites are heated above 500° C. topromote carbonization. Graphitization can then be promoted usingconventional state-of-the-art graphitization conditions.

Different synthesis procedures were used in the preparation of theethynylated aromatic hydrocarbons of this invention. These proceduresare illustrated in Table I in general form. In many cases, two or threemethods were used before the preferred method was identified. Method Awas used only for the synthesis of meta- and para-diethynylbenzene (DEB)from a mixture of divinylbenzene isomers, even though DEB isomers werealso produced successfully by the Vilsmeier process (Method B).

The polymerization of ethynylated aromatic hydrocarbons tends to proceedquite rapidly at or above 160°-180° C., but runaway reactions can alsooccur at even lower temperatures due to the liberated heat of reaction.Nevertheless, controlled polymerizations can be achieved at temperaturesbetween 100°-150° C. For example, controlled polymerization of a 50:50mixture of 1-ethynylpyrene and diethynylbenzene has also been achievedat 125° C. and a high quality 3D carbon-carbon composite was produced.At these lower temperatures, where polymerization is slow, pressure isrequired to prevent monomer evaporation.

Synthesis of small quantities of ethynylpyrene (EP) by the Method C wassuccessfully repeated several times. Subsequently, 1 kilogram wasprepared. This latter material melted at a slightly lower temperature(102°-108° C. versus 109°-113° C.), but its infrared spectrum indicatedthat it was identical to the original material. The difference inmelting range is attributed to a slightly different concentration ofisomers, probably due to a difference in solvents (nitrobenzene vs.methylene chloride) used in the synthesis. The 1 kilogram lot was usedfor carbon-carbon composite fabrication purposes.

                  TABLE I                                                         ______________________________________                                        METHOD A                                                                       ##STR2##                                                                      ##STR3##                                                                     HCCArCCH                                                                      METHOD B                                                                       ##STR4##                                                                      ##STR5##                                                                      ##STR6##                                                                     METHOD C                                                                       ##STR7##                                                                      ##STR8##                                                                      ##STR9##                                                                     ______________________________________                                    

In a typical diethynylbenzene-ethynylpyrene blend, thermally inducedcopolymerization occurs, initially producing polymers such as thefollowing: ##STR10##

Subsequently, the polymerization will proceed to yield an infinitenetwork of crosslinked resin. The char yield of such a copolymer isdependent upon the pressure at which charring was performed. It wasfound that a 125 psi char yield curve is approximately 28 percent higherthan an ambient pressure char yield curve when the same are compared.Char yields were measured by thermogravimetric analysis on thepolymerized blends of the ethynylated compounds. Rates of heatingaffected these yields to some extent, but not significantly so.

For comparison purposes, char yields were also measured on several coaltar pitches obtained from Ashland Oil Company. Yields at 800° C. areshown after pyrolysis in nitrogen. The values are shown in Table II.None of the pitches were outstanding char formers at ambient pressure.The one yielding 52 percent char was very high melting.

                  TABLE II                                                        ______________________________________                                          Pitch              Char                                                     ______________________________________                                        RD-131-R (low sulfur No. 240)                                                                      22%                                                      RD-130-R (No. 260)   30%                                                      RD-129-R (No. 240)   30%                                                      RD-132-R (No.210C)   52%                                                      RD-128-R (No. 170)   13%                                                      ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        CHAR YIELDS OF VARIOUS A-15 PITCH BLENDS                                      CONTAINING POLMERIZABLE ADDITIVE                                                          Additive     DTBP,   Char Yield at                                Additive    Concentration, %                                                                           %       80° C., %                             ______________________________________                                        Diethynylbenzene                                                                          20                   50-84                                                    33                   63                                                       50                   84-88                                        Divinylbenzene                                                                            100                  12                                                       20                   59                                                       33           3       61                                           Phenylacetylene                                                                           100                   2                                                       100                  6 after 215° C.                                                        post-cure                                    Diethynylbiphenyl                                                                         50                   81                                           ______________________________________                                    

Other char yield measurements were made on A-15 pitch blends containingadditives such as diethynylbenzene, divinylbenzene, phenylacetylene anddiethynylbiphenyl. In one case, ditertiary butyl peroxide was added tocatalyze polymerization. The blends studied are shown in Table III.

Carbon-carbon materials utilizing the resin matrix precursor of thisinvention were prepared as unidentified components. They weregraphitized, and their microstructure studied with a scanning electronmicroscope (SEM). The unidirectional composites were prepared byvacuum-pressure impregnating yarn bundles wrapped in a simple graphitefixture. These components were prepared in accordance with the followingschedule:

1. Wind dry yarn bundles on graphite fixture.

2. Vacuum-pressure impregnation with matrix pressure.

3. Polymerize the matrix at low temperature and pressure (usuallyambient atmospheric pressure and 120°-200° C.).

4. Graphitization to 2700° C.

Table IV lists the unidirectional specimens prepared and examined. Inthe examination procedure, small composite segments were cut from themandrel and prepared using standard metallographic techniques. Thepolished specimens were then oxygen plasma-etched, and examined atmagnifications from 20X to 10,000X. Graphitization, or lack of it, wasapparent in high magnification photomicrographs of the oxygen plasmaetched specimens; these studies were relied on to definegraphitizability. In some instances, a polymer would graphitize in thevicinity of fibers while not developing graphitic structure a few fiberdiameters away from the interface. These studies verified the conditionsof the graphitic or nongraphitic matrix in matrix pockets and in fiberbundles. DEB, EP and mixtures of the two were chosen for fabrication oftest billets. Table V shows the processing steps for the pure compoundsand mixture of EP and DEB.

An objective of this invention was predicated on improved carbon beingobtained under mild processing conditions. The selection of EP and DEBwas compatible with this objective. DEP and EP have carbon to hydrogenratios which are higher than those of most organic compounds.Experimental values have been obtained for comparison (Table VI). Sinceyield will depend upon experimental conditions, temperatures andpressures at which the data were obtained are included. Commonly, morethan 95% of the weight loss occurs below 850° C. Most of the informationpresented was obtained in two-step processing in which case the productyield fractions for each step gives the overall yield of that materialto the indicated temperature.

                                      TABLE IV                                    __________________________________________________________________________    UNIDIRECTIONAL COMPOSITES                                                                       Ultimate                                                                      Cure State of  State of                                     Specimen          Temp Polymerized Mass                                                                        Graphitized                                  Number                                                                             Material     °C.                                                                         (as received)                                                                           Mass                                         __________________________________________________________________________    1    Unidirectional composite                                                                   300  Coherent composite                                                                      Coherent                                          with 1-ethynylpyrene                                                                            structure                                                   matrix                                                                   2    Undirectional composite                                                                    300  Coherent composite                                                                      Generally                                         20% 1,3-diethynylbenzene                                                                        structure coherent, some                                    in 1-ethynylpyrene          delamination                                                                  (melting or de-                                                               polymerization                                                                occurred during                                                               graphitization                                                                processing)                                  3    Unidirectional composite                                                                   300  Coherent composite                                                                      Coherent                                          20% 1,3-diethynylbenzene                                                                        structure                                                   in 1-ethynylpyrene matrix                                                     (pressure cured)                                                         __________________________________________________________________________

                  TABLE V                                                         ______________________________________                                        DEB PROCESSING                                                                 ##STR11##                                                                    EP PROCESSING                                                                  ##STR12##                                                                    EP & DEB MIXTURE PROCESSING                                                    ##STR13##                                                                

                  TABLE VI                                                        ______________________________________                                        CARBON YIELDS OF 1,3-DIETHYNYLBENZENE (DEB)                                   AND 1-ETHYNYLPYRENE (EP):                                                     THEORETICAL AND EXPERIMENTAL                                                               Experi-                                                                              Processing Conditions                                            Theoretical                                                                           mental   Temp. °C.                                                                        Pressure (psi)                              ______________________________________                                        DEB      95.2      94%      2700    15                                        EP       95.6      44%      300     75                                        20% DEB: 95.5 76%  850      75                                                80% EP                                                                        33% DEB: 95.4      89%      850     125                                       67% EP                                                                        ______________________________________                                    

                  TABLE VII                                                       ______________________________________                                        CARBON GRAPHITE YIELD VERSUS PYROLYSIS                                        PRESSURE FOR 15-V PITCH (T.sub.F = 2700° C.)                                  Pressure                                                                             Yield                                                                  (psi)  (%)                                                             ______________________________________                                                15    24                                                                     300    65                                                                     920    82                                                                     5,000  82                                                                     15,000 80-84                                                           ______________________________________                                    

For example, the 20DEB; 80EP mixture was first cured at 75 psi and 300°with a 96.1% yield: then the resulting polymer was heated to 850° C. at75 psi with a 79% yield for that process. The product of those yieldfigures gave the tabulated value of 75% (850° C., 75 psi). Forcomparison, Table VII is included to show the state of pitch processingtechnology for Allied Chemical 15V pitch. The mechanism of decompositionis very important both with pitch and with these candidate precursormaterials. With DEB, little or no gas evolution was observed duringpolymerization and a high overall carbon yield was obtained with lowpressure processing. With pure EP this is not the case; low pressurechar was very porous, the polymerization process apparently entrappingbubbles of gases evolved during the polymerization reaction. Ambientpressure cure gave a low yield. EP resembles pitch in thischaracteristic. Mixtures of EP and DEB gave yields of about 80% carbonat 300 psi or lower while 900 psi pressure is required to achieve thatpercentage yield with pitch. It is concluded that most improvement ofyield in a mixture occurs by adding 40 or 50 weight percent of DEB toEP.

The compounds of this invention polymerize into a char precursorgraphitizable at low temperature. This feature determines the nature ofthe matrix pocket microstructure of the composite, and results in amaterial similar in structure to high pressure processed pitchcarbon/carbon. Experience shows this microstructure to have betterperformance characteristics in erosion resistance and ablation as wellas better basic mechanical and thermal properties which relate to thatperformance.

DEB's major disadvantage, when used alone as a matrix precursor is therequirement for critical temperature control during polymerization. Thelarger the mass of material being processed, the more difficult itbecomes to control the polymerization, especially when using a constanttemperature process. Inside a preform, this may not be as great aproblem as with the excess of liquid puddled on top of a preform, as wasrequired in these experiments, to assure complete impregnation of thecomposite. High vapor pressure and an associated low boiling point makeprocessing difficult, both as the pure compound and in a mixture. Tomaintain a given composition, time becomes critical; time of applicationof vacuum is held to a minimum, and hard vacuum is avoided.

EP when used alone, has some advantages: it shows only a very small ornonexistent exotherm in its polymerization, and it is easilygraphitized. Temperature control would be less of a problem if the purecompound could be processed. The graphitizability which carries overinto its use in a mixture makes it useful in matrix formation. Itsnormal state as a solid at room temperature complicates processing byrequiring elevated temperature impregnation. The melting point of 115°C. is not inordinately high, but does limit its working temperaturerange. Unfortunately, it has a comparatively low carbon yield and yieldsporous char.

The DEB-EP mixtures of this invention, however, have been selectedbecause of an apparent synergism which results in: (1) a reducedexotherm; (2) greater ease of graphitization; (3) improved carbon yieldrelative to EP alone, and dense char similar to that of DEB (andassociated absence of gas evolution); and finally, (4) polymerization toa set microstructural framework within matrix pockets. The matrixmaterial fortunately inherits the good characteristics of bothcomponents while minimizing the disadvantage of the pure compounds. Thesolubility of EP in DEB, increasing with increasing temperature, allowsa reasonable working range for impregnation with their mixture (70°-100°C.). Table V shows the processing steps for the pure compounds and amixture of EP and DEB.

Further studies to determine the effectiveness of the precursor materialof this invention were carried out using 3-dimensional PAN minibillets(1 cm cylinders) of PAN fibers cut from a larger 3-D preform.

These minibillets are identified as specimens 22, 23 and 24. Reactiontemperature limits were investigated by processing in a modified DIAapparatus in order to program the long, closely controlled temperaturecycles required for curing and pyrolyzing to 700° or 300° C. Samples ofthese billets were checked for weight pickup in single cycle andmultiple cycle processing. The required scale-up to larger equipment andlarger amounts of precursor material was done by adapting a UnitedStates autoclave which had vacuum to 300 psi pressure, capability aswell as reasonable temperature control. To maintain closer temperaturecontrol, samples were processed within containers which are insertedinto cavities within a large graphite block. This large mass was neededfor its thermal inertia to eliminate temperature cycling. Chromelalumelthermocouples inserted into two positions within the block indicatedthat constant and controllable temperature was maintained within ±1° C.over periods of 16 or more hours in the most recent processing cycles.

Two one-inch cubes (specimens 25 and 26) cut from the same PAN 3-Dpreform were partially processed in this fashion. The processingproceeded as depicted in the flow diagram of Table VIII. Water immersionmeasurements, conducted by ASTM procedure C20-46, were used to monitorprocessing and to determine the amount of precursor required to fill thebillet in the following cycle. An excessive amount of matrix precursoris undesirable because of the potential problem of a runaway reactionduring polymerization. In each cycle the billets were processed in aclose fitting steel container conserving expensive experimentalmaterials and minimizing the potential for exotherm. Processing detailsand billet characteristics are shown in Table IX. With the firstminibillet, specimen 25, pure DEB was used as the matrix precursor. Thesecond, specimen 26, was processed with a 50:50 weight percent mixtureof DEB and EP. ASTM C-20 data were not obtained for each step of theprocessing of this billet but the bulk density curve for densificationappeared to be reasonably good considering the exotherm experienced incycle 1 of the densification. Analysis based on data available showedthat because of the closed porosity developed in the first and secondcycles, this point represents a 90-100 percent efficiency in filling ofthe available (open) pore space.

Difficulties with DEB processing, and the consideration discussed above,led to the use of the mixture of compounds used with billet 26. Thisbillet processed reasonably well, but the density curve and porositydata show that if continued on the same basis as the first four cycles,it probably would not fully densify. The reason for this has beenobserved in the nature of the char which developed. Gas evolution, whichapparently occurs during polymerization of the EP component when it is alarge fraction of the total mixture, causes the char to have low densityand some closed porosity. This may have resulted from evaporation of theDEB component of the mixture because of two factors: the large openvolume of the autoclave and the vacuum processing of the DEB componentwith its high vapor pressure.

                  TABLE VIII                                                      ______________________________________                                         ##STR14##                                                                     ##STR15##                                                                    ______________________________________                                    

                                      TABLE IX                                    __________________________________________________________________________    3-D BILLETS PREPARED DURING PROCESS DEVELOPMENT                               Specimen        Matrix No. of                                                                            Graphitization Temperature                         No.  Type                                                                              Size   Precursor                                                                            Cycles                                                                            (°C.)                                       __________________________________________________________________________    22   3-D 1 cm cylinder                                                                        DEB    2   2600 (fast heat ramp, no                                PAN                   hold at T.sub.gr)                                  23   3-D 1 cm cylinder                                                                        2 EP:1 DEB                                                                           1   2600 (slow heat ramp, 1/2 hr                            PAN                   hold at T.sub.gr)                                  24   3-D 1 cm cylinder                                                                        DEB    1   2600 (slow heat rame, 1/2 hr                            PAN                   hold at T.sub.gr)                                   25* 3-D 1 × 1 × 1 in.                                                            DEB    2   2600                                                    PAN cube                                                                 26   3-D 1 × 1 × 1 in.                                                            1 EP:1 DEB                                                                           4   27,00 2300                                              PAN cube                                                                  27* 3-D 2 × 2 × 3 in.                                                            1 EP:1 DEB                                                                           2   2700                                                    PAN                                                                      __________________________________________________________________________     *A preliminary pyrolysis was at too high a temperature and resulting in a     exotherm witout any apparent damage to the billet.                       

The process development work on chars of pure compounds, unidirectionalminibillets, and small 3D composites were carried out to prepare forfabrication of a modest sized billet which would be cut into testspecimens (mechanical, erosion, ablation) to obtain data for comparisonwith data on carbon-carbon materials processed by standard methods suchas HiPIC EISP processes for missile nosetips. The 2 in.×2 in.×3 in.billet Specimen 27 provided sufficient material for characterization.The larger size of this billet necessarily involved further scale-up inthe use of matrix precursor. In the first impregnation-polymerizationcycle, the temperature program which proved satisfactory for 1 inchcubes was not suitable for the larger billet as processed. The reactionran away, as the temperature of the chamber reached 135° C., resultingin a low density porous char. The apparent reason for the runawayreaction is that with the larger mass of liquid precursor the exothermicheat developed on polymerization was not conducted away through theliquid fast enough to prevent a continually increasing temperature. Toavoid the exotherm, either (1) polymerization must be accomplished at alow temperature for a longer time, or (2) the amount of excess monomerused must be reduced. The former conditions were used for the secondimpregnation of billet 27. The temperature of polymerization, previously135°-145° C. for small billets, was reduced to 123° C.

To summarize the present invention, it has been found that a new lowpressure impregnated and carbonized carbon/carbon composite can beproduced from a mixture of high char yielding acetylenic precursormaterials; namely, a copolymerizable mixture of 1,3-diethynylbenzene andethynylpyrene. Processing methods have been developed which allowdensification to 1.8 g/cm³. The partial polymerization step is done atlow pressure and low temperature (300 psi, 123° C.) over a period ofabout 16 hours. The material is comparable to HiPIC densifiedcarbon/carbons in microstructure, mechanical properties, (except shear)and thermal expansion. The advantages of this material are: HiPIC-likemicrostructure from a low pressure process; thermomechanical propertiescomparable to HiPIC composites; reproducible matrix material composition(synthetic precursor of low impurity content, therefore, multisourceacquisition and improved quality control are possible); reduction inprocessing cycles from other LoPIC processing to achieve comparabledensity (high char yield); and potential for development of much largercomposites because of low pressure processibility.

While the invention has been described with particularity in referenceto specific embodiments thereof, it is to be understood that thedisclosure of the present invention is for the purpose of illustrationonly and is not intended to limit the invention in any way, the scope ofwhich is defined by the appended claims.

What is claimed is:
 1. A homogeneously compounded composition comprisinga resinous copolymer of diethynylbenzene and ethynylpyrene.
 2. Acomposition in accordance with claim 1 wherein said diethynylbenzene andsaid ethynylpyrene are present in a monomer weight ratio of betweenabout 1:9 to 9:1.
 3. A composition in accordance with claim 1 whereinsaid diethynylbenzene and said ethynylpyrene are present in a monomerweight ratio of between about 1:4 to 1:1.
 4. A composition in accordancewith claim 1 wherein said diethynylbenzene and said ethynylpyrene arepresent in a monomer weight ratio of about 1:1.
 5. A process forsynthesizing an aromatic acetylene copolymeric compound which comprisesthe steps of (1) forming a mixture of diethynylbenzene andethynylpyrene; (2) heating said mixture to a temperature within therange of about 120° to 140° C. at a pressure of about 300 psi; (3)continuing to heat said mixture under pressure for a period of timesufficient to effect the copolymerization of said mixture; and (4)separating the resulting reaction product.
 6. A process in accordancewith claim 5 wherein said mixture is heated to a temperature of fromabout 123° to about 130° C. at a pressure of 300 psi.
 7. A process inaccordance with claim 5 wherein said mixture is heated to a temperatureof 123° C. at a pressure of 300 psi for a period of about 16 hours.